Attractive properties can be obtained from ceramic composites having a texture in which the distribution of two or more materials are well controlled. An example of such textured ceramic composites are fibrous monolithic ceramics. Unlike ordinary ceramics which abruptly and catastrophically suffer tensile fracture, fibrous monoliths have the unique property of non-brittle fracture--they gracefully split and delaminate, like, e.g., wood, thereby providing for non-catastrophic failure. This property is of great value in many applications, e.g., high temperature structural applications such as those encountered by engine components as well as a number of other automotive structural applications.
A more detailed description of the structure and properties of such fibrous monolithic ceramics is provided in U.S. Pat. No. 4,772,524. This patent discloses a fibrous monolithic ceramic body as comprising a plurality of compacted, coated, and sintered fibers. These fibers comprise a core of a first ceramic composition, and a coating on that core of a different ceramic composition. This coating is referred to as a "debond phase," and serves as a "plane of weakness." The particular debond phase described in the '524 patent is said to be comprised of three ceramics--aluminum titanate, zirconia, and halfnia--all of which possess a tendency to spontaneously microcrack. By providing a layer of these microcracked ceramics, it was found that the desired "plane of weakness" was formed in the fiber.
It is this plane of weakness which provides a fibrous monolith prepared using such a fiber, after sintering, with a non-brittle fracture characteristic. Specifically, the interface, which defines a plane of weakness, will function to deflect a crack in the coating, or "debond phase, from normal to the plane of weakness to a direction parallel to the plane of weakness. Thus, catastrophic failure of the fibrous monolith prepared using such fibers is avoided.
The '524 patent further discloses a process for preparing such fibrous monolithic ceramics. This process comprises coating a fugitive cotton thread by passing that thread first through a suspension of the core composition, and subsequently through the coating composition, to provide a ceramic fiber. These fibers are then arranged together to form the desired fibrous monolith.
Since the issuance of the '524 patent, new varieties of fibrous monolithic ceramics have been discovered. See, e.g., S. Baskaran et al., "SiC-Based Fibrous Monolithic Ceramics," Ceramic Sci. & Eng. Proc. 14 (9-10) pp. 813-823; S. Baskaran et al., "Fibrous Monolithic Ceramics, I: Fabrication, Microstructure, and Indentation Behavior," J. Am. Cer. Soc'y 76 (9), pp. 2209-16 (1993); S. Baskaran et al., "Fibrous Monolithic Ceramics, II: Flexural Strength and Fracture Behavior of the SiC/Graphite System," J. Am. Cer. Soc'y 76 (9) pp. 2217-24 (1993); S. Baskaran et al., "Fibrous Monolithic Ceramics, III: Mechanical Properties and Oxidation Behavior of the SiC/BN System," J. Am. Cer. Soc'y 77 (5) pp. 1249-55 (1994); S. Baskaran et al., "Fibrous Monolithic Ceramics, IV: Mechanical Properties and Oxidation Behavior of the Alumina/Ni System," J. Am. Ceramic Soc'y, 77, (5) pp. 1256-62 (1994); and D. Popovic' et al., "Silicon Nitride and Silicon Carbide Fibrous Monolithic Ceramics" 42 Silicon Based Structural Ceramics (B. W. Sheldon et al. eds., Am. Cer. Soc'y, Westerville, Ohio, 1994) pp. 173-86. In these newly discovered ceramic fibrous monoliths, the ceramic fibers from which they are prepared establish a plane of weakness therein by using a graphite layer or a boron nitride layer. The core composition, in contrast, was able to be prepared from a wide variety of ceramics including, e.g., silicon carbide, silicon nitride, and alumina.
In conjunction with or shortly after the discovery of the aforementioned new materials, new methods for preparing the ceramic fibers used to fabricate fibrous monoliths were also discovered. Specifically, it was taught that the core of a green ceramic fiber could be prepared either by dry spinning or melt spinning a composition comprising a polymer and ceramic powder. To complete the ceramic fiber, it was further taught that the coating layer was to be subsequently applied by dipping the core into a slurry of the debond phase composition.
Three U.S. patents have issued which involve the extrusion of a mixture of a ceramic powder and a polymer to form a fiber. The first patent, U.S. Pat. No. 4,908,340, discloses the extrusion of ceramic green fibers by melt spinning a mixture of thermoplastic polymers and ceramic powders. The second, U.S. Pat. No. 4,990,490, describes a process for the thermoplastic extrusion of green fibers from superconducting ceramics which are subsequently coated with metal powders. The third patent, U.S. Pat. No. 5,041,248 describes the extrusion of green fibers by melt spinning polyisobutylene with ceramic powders. This patent further discloses that its thermoplastic extrusion process may be used to make sintered ceramic bars, rods, tubing, or fibers from ceramic-polymer mixtures. The mixtures are described as those in which the polymer acts as a fugitive vehicle, it being later removed during the heat treatment required to obtain a sintered ceramic product.
In summary, fibrous monoliths have traditionally been fabricated using fibers that were prepared by the laborious process of dip-coating previously extruded solid ceramic cores in a coating composition comprising ceramics and polymers. While this provides a textured fiber, it is slow, inconvenient to set-up and use, difficult to control, and is unable to provide a uniformly-textured fiber.
Thus, there exists a need for a more efficient method for preparing fibrous monolithic ceramics which exhibit non-brittle fracture characteristics using green ceramic fibers. The exists a further need for a method by which the texture of fibers used to prepare such monoliths can be more readily controlled.
It is therefore an object of the present invention to provide a relatively efficient method for preparing fibrous monolithic ceramics which exhibit non-brittle fracture characteristics from green ceramic fibers.
Another object of the present invention is to provide a relatively efficient method for preparing such green ceramic fibers despite the presence of high levels of ceramic particulate loading in any composition from which the fibers are prepared.
A further object of the present invention is to provide a green ceramic fiber useful for preparing fibrous monolithic ceramics in which the texture of the fiber is precisely controllable within defined parameters.
Yet another object of the present invention to provide a method for increasing the strength of fibrous monolithic ceramics.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.